As is well known, chromatography is based on the principle that different substances within a mixture can be separated from one another and concentrated into zones by passing the mixture through a two phase system. One phase of the system, such as a gas or liquid phase, acts as a carrier for the mixture while the other phase, such as solid granular absorbent powder, exerts a differential restraining force on the components of the mixture to cause separation thereof.
When using chromatographic columns to separate closely related complex substances, a sample of the material to be separated is fed through the granular absorbent packed within a long column. As the sample flows through the column the different components thereof separate and stratify so that they reach the exit end of the column in a sequential fashion. Such columns normally consist of tubes, such as stainless steel tubes, that are tightly packed with a very fine grain powdered material having a large specific surface area. Exposure of the sample to this large specific surface area causes some components of the sample to be restrained in their flow, thereby permitting the column to provide distinct and reproducible separation and resolution of the sample into its component parts as it travels through the column.
In order to obtain highly efficient separation or fractionation of the component parts of the sample, particularly where one component is present in only a small amount and is close to a major component, it is essential to provide uniform exposure of the sample across the entire cross-sectional face of the column. Maximum efficiency of a chromatographic column is obtained when the sample enters the packed column with a uniform flow profile across the entire face of the column and when the profile proceeds through the column at a uniform rate and when the carrier and sample progress in substantially a stratified or laminar fashion. If the sample is not uniformly spread across the face of the column, but instead is concentrated at the axis of the tube, it tends to exhibit a highly arcuate or crescent-shaped meniscus profile. In that event it will take longer to remove individual fractions at the exit end of the column and intermingling of the stratified layers may occur.
Additionally, at the exit end of the column, it is desirable to have the minimum possible volume between the end of the column packing and the exit tube leading to the chromatographic analyzer. Each increment of volume in the exit passage contributes to the widening of the detection band on the chromatographic read out and tends to obscure trace element peaks that may be small with respect to a major component of the sample and may occur at closely spaced locations from a major component peak in the chromatographic read out.
Conventionally, the packed chromatographic columns are sealed at each end with porous disc, usually stainless steel, placed at the very end of the column tube. One common method is to press the discs into a terminating assembly or fitting. Another technique provides a counter bore at the end of the column and the terminator disc elements are mounted directly into the counter bore at the end of the tube. In either event, the inlet and outlet tubes of the chromatographic column typically have a very small bore relative to the inside diameter of the chromatographic column and it is necessary to provide some means of distributing the sample entering the column and of collecting the sample exiting the column. As mentioned, it is extremely important to provide a uniform flow profile over the entire face of the column both at the entrance end of the column from the very small bore of the inlet flow tube and also at the outlet or exit end of the column. Heretofore this distribution was accomplished through the the use of a small screen that permitted lateral flow between the wires of the screen. These screens were required in addition to the porous discs that were used to retain the packing within the chromatographic columns.